[Technical Field]
5 [0001]
The present invention relates to a hot-dip plated steel sheet, and specifically, to a
hot-dip plated steel sheet in which defects are inconspicuous. Priority is claimed on
Japanese Patent Application No. 2020-061206, filed March 30, 2020, the content of
which is incorporated herein by reference.
10 [Background Art]
[0002]
Hot-dip plated steel sheets are used as steel sheets having favorable corrosion
resistance. Galvanized steel sheets, which are typical examples of hot-dip plated steel
sheets, are widely used in various manufacturing industry fields such as automobiles,
15 home appliances, and building materials.
[0003]
As a method of producing a galvanized steel sheet, a production method using a
cold-rolled steel sheet or a hot-rolled steel sheet as a base steel sheet, and passing it
through a continuous hot-dip galvanizing line (hereinafter referred to as CGL) is a usual
20 method. As a CGL process, a total reduction furnace method in which, in a cleaning
section on the entry side, a base steel sheet is degreased by alkaline spray and then
cleaned with a brush, and in an annealing section, annealing is performed in a reducing
atmosphere and immersion in a hot-dip galvanizing bath is then performed is generally
used. In addition, a Sendzimir method in which a non-oxidizing furnace is provided at a
25 stage previous to an annealing section, and a base steel sheet whose surface is cleaned is
1
pre-heated in the non-oxidizing furnace and then reduced and annealed in a reduction
furnace, and then immersed in a hot-dip galvanizing bath may be used.
[0004]
In order to further improve the corrosion resistance of the galvanized steel sheet
5 produced in the above process, a galvanized steel sheet having high corrosion resistance
in which a hot-dip galvanizing layer contains Al or Mg has been proposed. For
example, Patent Document 1 proposes a Zn-Al-Mg-based hot-dip plated steel sheet. In
addition, Patent Document 1 describes that a hot-dip plated steel sheet having better
corrosion resistance is obtained when a Zn-Al-Mg-based hot-dip plated steel sheet
10 contains one or more of Ca, Be, Ti, Cu, Ni, Co, Cr, and Mn.
[0005]
Incidentally, the Zn-Al-Mg-based hot-dip plated steel sheet mainly contains the
4 types of phases and structures of [Al phase], [Zn phase], [MgZnz phase], and [ternary
eutectic structure of AVMgZnz/Zn] in a hot-dip plated layer. In addition, when the hot-
15 dip plated layer contains Si in addition to Zn, Al, and Mg, it is mainly composed of 5
types of phases and structures including [MgzSi phase] in addition to the above 4 types of
phases and structures. Among these, the [Al phase] exhibits a white color when it
appears on the surface of the plating layer, and the [ternary eutectic structure of
AVMgZnz/Zn] exhibits a metallic luster. Since the [Al phase] and the [ternary eutectic
20 structure of AVMgZnz/Zn] are present on the surface of the plating layer in a mixed
manner, the surface of the hot-dip plated layer has a satin-like appearance.
[0006]
The satin-like appearance of the hot-dip plated layer is affected by the size of the
[Al phase] and the size of the [ternary eutectic structure of AVMgZnz/Zn]. If the sizes
25 of these phases and structures are substantially uniform over the entire surface of the hot-
2
5
dip plated layer, the overall appearance uniformity is improved. In recent years, various
techniques for improving the appearance of the hot-dip Zn-Al-Mg-based hot-dip plated
steel sheet have been proposed (Patent Documents 2 to 4).
[0007]
However, the Zn-Al-Mg-based hot-dip plated steel sheet with an improved
appearance has a problem that defects become more conspicuous when surface defects
occur during handling of the steel sheet. In particular, if the satin-like appearance of the
Zn-Al-Mg-based hot-dip plated steel sheet is used as a part of the product design, there is
a problem that defects are conspicuous.
10 [Citation List]
[Patent Document]
[0008]
[Patent Document 1]
PCT International Publication No. WO 2000/071773
15 [Patent Document 2]
20
Japanese Unexamined Patent Application, First Publication No. 2001-295015
[Patent Document 3]
Japanese Patent No. 4542434
[Patent Document 4]
Japanese Patent No. 5482914
[Summary of the Invention]
[Problems to be Solved by the Invention]
[0009]
The present invention has been made in view of the above circumstances, and an
25 object of the pr esent invention is to provide a hot -dip plated steel sheet in which defects
3
are inconspicuous and which has excellent corrosion resistance.
[Means for Solving the Problem]
[0010]
In order to address the above problem, the present invention uses the following
5 configurations.
10
15
[1] A hot-dip plated steel sheet including a steel sheet and a hot-dip plated layer
formed on a surface of the steel sheet,
wherein an average composition of the hot-dip plated layer contains Al: 2 to 22
mass%, and Mg: 0.1 to 10 mass%, with a remainder including Zn and impurities, and
wherein, when five square measurement areas with sides of 5 mm are selected
from a surface of the hot-dip plated layer and intensity ratios A for each measurement
area are obtained by a following measurement method, at least one ratio (A/Aave) of an
intensity ratio A of each measurement area to an average value Aave of intensity ratios A
of the five measurement areas is 0.50 to 0.65 or 1.45 to 2.00, and
wherein the measurement method is a method in which each measurement area
is extracted as 256 level grayscale image data with a size of 50 pixelsx50 pixels, a twodimensional
discrete Fourier transform is performed on the 256 gradation image data to
obtain amplitude spectrum images of spatial frequencies, and in the obtained amplitude
spectrum images, a sum S25 of intensities of spatial frequencies 1 to 25 and a sum S5 of
20 intensities of spatial frequencies 1 to 5 are calculated, and a ratio (S5/S25) of the
25
intensity sum S5 to the intensity sum S25 is defined as the intensity ratio A.
[2] The hot-dip plated steel sheet according to [1], wherein the average
composition of the hot-dip plated layer contains Al: 4 to 22 mass% and Mg: 1.0 to 10
mass%.
[3] The hot-dip plated steel sheet according to [1] or [2], wherein the average
4
5
composition of the hot-dip plated layer further contains Si: 0.0001 to 2 mass%.
[4] The hot-dip plated steel sheet according to any one of [1] to [3], wherein the
average composition of the hot-dip plated layer further contains a total of 0.0001 to 2
mass% of one or more of the group consisting of Ni, Ti, Zr, and Sr.
[5] The hot-dip plated steel sheet according to any one of [1] to [4], wherein the
average composition of the hot-dip plated layer further contains a total of 0.0001 to 2
mass% of one or more of the group consisting of Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr,
Sc, Y, REM, and Hf.
[6] The hot-dip plated steel sheet according to any one of [1] to [5], wherein an
10 adhesion amount of the hot-dip plated layer is a total of 30 to 600 g/m2 on both sides of
the steel sheet.
[Effects of the Invention]
[0011]
According to the present invention, it is possible to provide a hot-dip plated steel
15 sheet in which defects are inconspicuous and which has excellent corrosion resi stance.
[Brief Description of Drawings]
[0012]
Fig. 1 i s a diagram showing a typical cross-sectional SEM observation image of
a hot-dip plated steel sheet having a hot-dip plated layer having an average composition
20 containing Al: 11 mass%, Mg: 3 mass%, Si: 0.2 mass%, with the remainder including Zn
and impurities.
Fig. 2 i s a diagram showing an example of 256 level grayscale image data in a
measurement area and an example of an amplitude spectrum image of a spatial frequency
obtained by performing a two-dimensional discrete Fourier transform on the gradation
25 image data.
5
5
Fig. 3 is a diagram showing 256level grayscale image data items in
measurement areas and amplitude spectrum images of spatial frequencies obtained by
performing a two-dimensional discrete Fourier transform on the gradation image data
items.
Fig. 4 is a schematic diagram showing an example of a hot-dip plating facility
for a hot-dip plated steel sheet of the present embodiment.
[Embodiment(s) for implementing the Invention]
[0013]
A hot-dip plated layer having an average composition containing Al: 2 to 22
10 mass% and Mg: 0.1 to 10 mass% is mainly composed of 4 types of phases and structures
of [Al phase], [Zn phase], [MgZn2 phase], and [ternary eutectic structure of
AVMgZn:z/Zn]. In addition, when the hot-dip plated layer contains Si in addition to Zn,
Al, and Mg, it is mainly composed of 5 types of phases and structures including [Mg2Si
phase] in addition to the above 4 types of phases and structures.
15 [0014]
The [Al phase] exhibits a white color when it appears on the surface of the
plating layer, and the [ternary eutectic structure of Al/MgZn:z/Zn] exhibits a metallic
luster. Since the [Al phase] and the [ternary eutectic structure of AVMgZn:z/Zn] are
present on the surface of the plating layer in a mixed manner, the surface of the hot -dip
20 plated layer has a satin-like appearance. Thus, the satin-like appearance of the hot-dip
plated layer is affected by the size of the [Al phase] and the size of the [ternary eutectic
structure of AVMgZn:z/Zn]. If the sizes of these phases and structures are substantially
uniform over the entire surface of the hot-dip plated layer, the overall appearance
uniformity is improved.
25 [0015]
6
However, if physical defects (scratch marks, friction marks, etc.) occur on the
surface of the hot-dip plated layer when the appearance uniformity is improved, a new
problem in which defects are conspicuous occurs. In addition, the appearance of the
hot-dip plated layer is improved as the satin pattern becomes uniform, but the
5 improvement of the plating appearance is not limited to this, and for example, the
disordered appearance of patterns of various sizes may also improve the overall
appearance, as in a zinc plated steel sheet having a spangle pattern.
[0016]
Therefore, the inventors conducted extensive studies and found that, when
10 image data of the surface of the hot-dip plated layer is acquired, and image analysis is
performed by a two-dimensional discrete Fourier transform on the brightness of pixels
constituting the image data, it is possible to objectively evaluate the inconspicuousness of
defects in the hot-dip plated steel sheet as the disorder of patterns, and found that it is
possible to identify a hot-dip plated steel sheet in which defects are less conspicuous by
15 this evaluation method.
[0017]
Specifically, it is found that, when a plurality of areas with a predetermined size
are selected from the surface of the hot-dip plated layer, grayscale image data is acquired
for each area, a two-dimensional discrete Fourier transform is performed on these image
20 data items, and the obtained amplitude spectrum images of spatial frequencies are
analyzed, the analysis results of the amplitude spectrum images and the size of the satinlike
pattern are correlated, and additionally, when the variation of the analysis results of
the amplitude spectrum images is evaluated, it is possible to identify the disorder of
satin-like patterns, that is, inconspicuousness of defects.
25 [0018]
7
Hereinafter, the hot-dip plated steel sheet of the present embodiment will be
described.
[0019]
The hot-dip plated steel sheet of the present embodiment includes a steel sheet
5 and a hot-dip plated layer formed on the surface of the steel sheet, and the average
composition of the hot-dip plated layer contains Al: 2 to 22 mass%, and Mg: 0.1 to 10
mass%, with the remainder including Zn and impurities, and when five square
measurement areas with sides of 5 mm are selected from the surface of the hot-dip plated
layer and an intensity ratio A of each measurement area is obtained by the following
10 measurement method, at least one ratio (A/Aave) of an intensity ratio A of each
measurement area to an average value Aave of intensity ratios A of the five measurement
areas is 0.50 to 0.65 or 1.45 to 2.00 in the hot-dip plated steel sheet.
[0020]
In the measurement method, each measurement area is extracted as 256level
15 grayscale image data with a size of 50 pixelsx50 pixels, and a two-dimensional discrete
Fourier transform is performed on the 256 gradation image data to amplitude spectrum
images of spatial frequencies . In the amplitude spectrum images, a sum S25 of
intensities of spatial frequencies 1 to 25 and a sum S5 of intensities of spatial frequencies
1 to 5 are calculated, and a ratio (S5/S25) of the intensity sum S5 to the intensity sum
20 S25 is defined as an intensity ratio A in the method.
[0021]
The material of the steel sheet used as the base of the hot-dip plated layer is not
particularly limited. As the material, general steel or the like can be used without
particular limitation, Al killed steel or some high alloy steels can also be applied, and the
25 form is not particularly limited. When a hot-dip plating method to be described below
8
is applied to the steel sheet, the hot-dip plated layer according to the present embodiment
is formed.
[0022]
Next, the chemical composition of the hot-dip plated layer will be described.
5 The average composition of the hot-dip plated layer contains Al: 2 to 22 mass%, and Mg:
0.1 to 10 mass%, with the remainder including Zn and impurities. More preferably, the
average composition of the hot-dip plated layer contains Al: 2 to 22 mass%, and Mg: 0.1
to 10 mass%, with the remainder including Zn and impurities. In addition, the average
composition of the hot-dip plated layer may contain Si: 0.0001 to 2 mass%. In addition,
10 the average composition of the hot-dip plated layer may contain a total of 0.0001 to 2
mass% of one or more of the group consisting of Ni, Ti, Zr, and Sr. In addition, the
average composition of the hot-dip plated layer may contain a total of 0.0001 to 2 mass%
of one or more of the group consisting of Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y,
REM, and Hf.
15 [0023]
The Al content in the average composition is in a range of 2 to 22 mass%. Al
may be contained in order to secure corrosion resistance. If the Al content in the hot -dip
plated layer is 2 mass% or more, an effect of improving corrosion resistance is further
improved. If the Al content exceeds 22 mass%, the corrosion resistance decrease
20 although the cause is unknown. If the Al content exceeds 22 mass%, an effect of
improving corrosion resistance is saturated. In consideration of corrosion resistance, the
Al content is preferably 4 to 22 mass%, and more preferably 5 to 18 mass%. The Al
content is still more preferably 6 to 16 mass%.
[0024]
25 The Mg content in the average composition is in a range of 0.1 to 10 mass%.
9
Mg can be contained in order to improve corrosion resistance. If the Mg content in the
hot-dip plated layer is 0.1 mass% or more, an effect of improving corrosion resistance is
further improved. If the Mg content exceeds 10 mass%, the occurrence of dross in the
plating bath becomes significant, and there are locations in which the plating may not be
5 formed normally due to adhesion of dross to the plating so that the corrosion resistance
may decrease. Therefore, the Mg content is 10 mass% or less. In consideration of a
decrease of corrosion resistance due to the occurrence of dross, the Mg content is
preferably 1.0 to 10 mass%, and more preferably 1.5 to 6.0 mass%. The Mg content is
still more preferably in a range of 2.0 to 5.0 mass%.
10 [0025]
In addition, Si may or may not be contained because it may improve the
adhesion of the hot-dip plated layer. Since an effect of improving adhesion is exhibited
if the Si content is 0.0001 mass% or more, the Si content is preferably 0.0001 mass% or
more. On the other hand, the Si content is set to 2 mass% or less because an effect of
15 improving plating adhesion has already been saturated when the Si content exceeds 2
mass%. In consideration of plating adhesion, the Si content may be in a range of 0.0001
to 1 mass% or in a range of 0.01 to 0.8 mass%.
[0026]
The average composition of the hot-dip plated layer may contain a total of
20 0.0001 to 2 mass% of one or more ofNi, Ti, Zr, and Sr. In addition, the average
composition of the hot-dip plated layer may contain a total of 0.0001 to 2 mass% of one
or more of Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, and Hf. When these
elements are contained, it is possible to further improve corrosion resistance. REM is
one or more of rare earth elements having an atomic number of 57 to 71 in the periodic
25 table.
10
[0027]
The remainder of the chemical composition of the hot-dip plated layer is made
up of zinc and impurities. The impurities include those that are inevitably contained in
base metals such as zinc and those that are contained when steel is melted in a plating
5 bath.
[0028]
Here, the average composition of the hot-dip plated layer can be measured by
the following method. First, a surface layer coating film is removed with a coating film
remover (for example, Neo Rever SP-751 commercially available from Sansai Kako Co.,
10 Ltd.) that does not erode the plating, and the hot-dip plated layer is then dissolved with
hydrochloric acid containing an inhibitor (for example, Hibiron commercially available
from SUGIMURA Chemical Industrial Co., Ltd.), the obtained solution is subjected to
inductively coupled plasma (ICP) emission spectroscopy analysis, and thus the average
composition can be determined. In addition, when the surface layer coating film is not
15 provided, an operation of removing the surface layer coating film can be omitted.
[0029]
Next, the structure of the hot-dip plated layer will be described. Specifically,
each structure will be exemplified using a case in which the average composition of the
hot-dip plated layer contains Al: 11 mass%, Mg: 3 mass%, Si: 0.2 mass%, with the
20 remainder including Zn and impurities (Fig. 1). The hot-dip plated layer containing Al,
Mg and Zn contains the [Al phase] and the [ternary eutectic structure of Al/Zn/MgZn2].
It has a form in which the [Al phase] is contained in the base of the [ternary eutectic
structure of Al/Zn/MgZn2] . In addition, the [MgZn2 phase] and the [Zn phase] may be
contained in the base of the [ternary eutectic structure of Al/ZnJMgZn2]. In addition,
25 when the hot-dip plated layer contains Si, the [Mg2Si phase] may be contained in the
11
base of the [ternary eutectic structure of AVZn/MgZnz].
[0030]
As shown in Fig. 1, in the SEM image, the [ternary eutectic structure of
AVZn/MgZnz] is a part expressed by a white area, a gray area, and a fine white and black
5 mixed area. The [ternary eutectic structure of AVZn/MgZnz] is a ternary eutectic
structure of anAl phase, a Zn phase, and an intermetallic compound MgZnz phase, and
the Al phase forming the [ternary eutectic structure of AVZn/MgZnz] corresponds to, for
example, an "Al" phase" (anAl solid solution in which Zn is solid-solutionized and
containing a small amount ofMg) at a high temperature in the ternary Al-Zn-Mg
10 equilibrium state diagram. The Al" phase at a high temperature generally separately
appears as a fine Al phase and a fine Zn phase at room temperature. In addition, the Zn
phase in the [ternary eutectic structure of Al/Zn/MgZnz] is a Zn solid solution in which a
small amount of Al is solid-solutionized and a small amount of Mg is additionally solidsolutionized
in some cases. The MgZnz phase in the [ternary eutectic structure of
15 AVZn/MgZnz] is an intermetallic compound phase present in the vicinity of Zn: about 84
mass% in the Zn-Mg binary equilibrium state diagram. It is thought that, in each phase
shown in the state diagram, other additives elements are not solid-solutionized or are
solid-solutionized but only in a very small amount, and the amount thereof cannot be
clearly distinguished by general analysis. Therefore, the ternary eutectic structure
20 composed of these three phases is referred to as [ternary eutectic structure of
AVZn/MgZnz] in this specification.
[0031]
As shown in Fig. 1, the [Al phase] is a part represented with a state in which a
white color and a black color are finely mixed, which appears like islands with clear
25 boundaries in the base of the [ternary eutectic structure of AVZn/MgZnz] in the SEM
12
Image. The [Al phase] corresponds to, for example, an "Al" phase" (anAl solid
solution in which Zn is solid-solutionized and containing a small amount of Mg) at a
high temperature in the ternary Al-Zn-Mg equilibrium state diagram. In the Al" phase
at a high temperature, the amount of Zn and the amount of Mg that are solid-solutionized
5 differ depending on the concentrations of Al and Mg in the plating bath. The Al'' phase
at a high temperature generally separately appears as a fine Al phase and a fine Zn phase
at room temperature, but the island-like shapes appearing at room temperature can be
seen as a retention of the framework of the Al" phase at a high temperature. It is
thought that, in this phase seen in the state diagram, other additive elements are not solid-
10 solutionized or are solid-solutionized but only in a very small amount, and they cannot be
clearly distinguished by general analysis. Therefore, the phase derived from the Al"
phase at a high temperature and having a shape of retaining the framework of the Al"
phase is referred to as an [Al phase] in this specification. The [Al phase] can be clearly
distinguished from the Al phase forming the [ternary eutectic structure of AVZn/MgZn2]
15 in microscopic observation.
[0032]
As shown in Fig. 1, the [Zn phase] is a part represented with a white color,
which appears like islands with clear boundaries in the base of the [ternary eutectic
structure of AVZn/MgZn2] in the SEM image. An area having a circle-equivalent
20 diameter of 2.5 J.lm or more is defined as the Zn phase. In the [Zn phase], actually, a
small amount of Al and additionally, a small amount of Mg are solid-solutionized in
some cases. It is thought that, in this phase seen in the state diagram, other additive
elements are not solid-solutionized or are solid-solutionized but only in a very small
amount. This [Zn phase] can be clearly distinguished from the Zn phase forming the
25 [ternary eutectic structure of AVZn/MgZn2] in microscopic observation. The plating
13
layer of the present invention may contain the [Zn phase] depending on production
conditions. However, because almost no effect on the improvement of corrosion
resistance of a processed part was observed in the experiment, there is no particular
problem even if the plating layer contains the [Zn phase].
5 [0033]
As shown in Fig. 1, the [MgZnz phase] is a part represented with gray, which
appears like islands with clear boundaries in the base of the [ternary eutectic structure of
Al/Zn/MgZnz] in the SEM image. In the [MgZnz phase], actually, a small amount of Al
is solid-solutionized. It is thought that, in this phase seen in the state diagram, other
10 additive elements are not solid-solutionized or are solid-solutionized but only in a very
small amount. This [MgZnz phase] can be clearly distinguished from the MgZnz phase
forming the [ternary eutectic structure of Al/Zn/MgZnz] in microscopic observation.
The plating layer of the present invention may not contain the [MgZnz phase] depending
on production conditions, but the [MgZnz phase] is contained in the plating layer under
15 most production conditions.
[0034]
As shown in Fig. 1, the [MgzSi phase] is a part represented with black, which
appears like islands with clear boundaries in the solidified structure of the hot-dip plated
layer when Si is contained in the SEM image. It is thought that, in the [MgzSi phase]
20 seen in the state diagram, Zn, Al, and other additive elements are not solid-solutionized
or are solid-solutionized but only in very small amount. The [MgzSi phase] can be
clearly distinguished in microscopic observation in the plating.
[0035]
Next, the appearance of the hot-dip plated layer will be described.
25 In the hot-dip plated layer according to the present embodiment, when an
14
intensity ratio A for each measurement area is obtained by the following measurement
method for five measurement areas selected from the surface of the hot-dip plated layer,
it is necessary for at least one ratio (A/ Aave) of an intensity ratio A of each measurement
area to an average value Aave of intensity ratios A of the five measurement areas to be
5 0.50 to 0.65 or 1.45 to 2.00.
[0036]
The five measurement areas may be arbitrarily selected, but the distance
between the measurement areas is preferably, for example, 10 em or less. If the
distance between the measurement areas is more than 10 em, it is difficult to
10 appropriately determine the disorder of appearance patterns such as a satin pattern, and
there is a risk of inconsistency with the visual determination result of the
inconspicuousness of defects. When the inconspicuousness of defects is visually
determined, a range of 10 em square is visually determined in many cases in
consideration of the size of the pattern. Therefore, in the present embodiment, the
15 distance between the measurement areas is 10 em or less. More specifically, at any
position on the surface of the plating layer, a square area with a side of 10 em is selected,
and at a total of 5 positions, four corners of the square and the intersection of two
diagonal lines of the square, measurement square areas with sides of 5 mm may be
selected.
20 [0037]
25
A sample including the selected measurement area and its surroundings is cut
out from the hot-dip plated steel sheet, and image data of the measurement area is
extracted using this sample.
[0038]
The image data of the measurement area is extracted by scanning the surface of
15
the hot-dip plated layer of the sample with a scanner connected to a computer. The
measurement area is extracted as 256 level grayscale image data with a size of 50
pixelsx50 pixels. The scanner to be used may be, for example, a flat bed type scanner.
Generally, in the acquisition of the image data with a scanner, since image correction is
5 performed for each acquisition, it is preferable to extract the measurement area by
trimming after image data of the entire sample is acquired at one time. In addition, with
a general scanner, image data with a large number of pixels of 50 pixels or more is
acquired for 5 mm, it is better to re-size the image to 50 pixelsx50 pixels using computer
software.
10 [0039]
Generally, image data can be extracted by taking a picture, but in the case of
taking a picture, because it is difficult to emit the uniform illumination light for taking
the picture to the entire hot-dip plated layer that is a subject, and it may not be possible to
accurately evaluate the disorder of patterns, extraction with a scanner is preferable.
15 [0040]
In addition, the image data is 256level grayscale image data. The image data
includes a binarized image, a gradation image, a color image and the like, but the
binarized image is represented by two light and dark values, and the amount of
information is insufficient. In addition, in the color image, since pixels have color
20 information, the amount of information is excessive. Since the surface appearance of
the hot-dip plated layer of the present embodiment has low saturation, grayscale
gradation image data is sufficient as the amount of information. Therefore, in the
present embodiment, a 256 level grayscale image having an appropriate amount of
information and a gradation of 0 to 255 steps that is easy to handle by a computer is
25 preferable.
16
[0041]
In addition, when a measurement square area with a side of 5 mm is extracted as
gradation image data with a size of 50 pixelsx50 pixels, the gradation image data
includes 2,500 pixels. Each pixel will have brightness data in a square area with a side
5 of0.1 mm, which reflects the appearance pattern such as a satin pattern. Theoretically,
even if the size of the measurement area and the number of pixels increase, the same
tendency of measurement results can be obtained, but in order to reduce the calculation
load, a measurement area of 5 mm and 50 pixels, which is a minimum size at which there
are no practical issues, is set.
10 [0042]
Next, a two-dimensional discrete Fourier transform is performed on the obtained
gradation image data of the measurement area to obtain amplitude spectrum images of
spatial frequencies. The two-dimensional discrete Fourier transform may be performed
by a computer having a program installed in advance.
15 [0043]
The two-dimensional discrete Fourier transform is performed using the
following (Formula 1). f(x, y) is a pixel value at a position of (x, y), and F(u, v) is a
complex number indicating a sine wave at a position of a spatial frequency (u, v). In
addition, u is a frequency of the x component, and v is a frequency of the y component.
20 In the present embodiment, since gradation image data with a size of 50 pixelsx50 pixels
is used, M and N are 50. When the absolute value of the complex number indicating a
sine wave is obtained, an amplitude spectrum image is obtained. For the obtained
amplitude spectrum image, in order to improve ease of handling of data, an operation of
switching a first quadrant with a third quadrant and a second quadrant with a fourth
25 quadrant is performed.
17
[0044]
[Math. 1]
F( u, v) = ,~l I/( x,y )e ln{: + ~) (it 1 )
x=O y=O
Formula 1
5 [0045]
For example, when the pattern on the plating surface is a rough satin pattern, if a
two-dimensional discrete Fourier transform is performed, many sine waves with a
relatively small frequency component are included. On the other hand, when the pattern
on the plating surface is a fine satin pattern, if a two-dimensional discrete Fourier
10 transform is performed, many sine waves with a relatively large frequency component
are included. The amplitude spectrum images of spatial frequencies after the discrete
Fourier transform is performed reflect the results of such a discrete Fourier transform.
[0046]
Fig. 2 shows an example of gradation image data and an example of an
15 amplitude spectrum image of a spatial frequency obtained by performing a twodimensional
discrete Fourier transform on the gradation image data. The amplitude
spectrum image shows an amplitude of a sine wave with a lower frequency component as
it approaches the center of the image, and shows an amplitude of a sine wave with a
higher frequency component as it is away from the center concentrically. In the spatial
20 spectrum image shown in Fig. 2, the magnitude (intensity) of the amplitude of the sine
wave is represented by shading, with darker colors indicating higher intensity and lighter
colors indicating lower intensity. That is, the spatial spectrum image shown in Fig. 2
shows that the intensity of the sine wave having a low frequency component is high
18
among sine waves of all frequencies.
[0047]
Then, as shown in Fig. 2, in the amplitude spectrum image of spatial frequencies
for each gradation image data item, a sum S25 of intensities of spatial frequencies 1 to 25
5 and a sum S5 of intensities of spatial frequencies 1 to 5 can be calculated. Here, the
sum S25 of intensities of spatial frequencies 1 to 25 or more is a sum of intensities in the
area surrounded by the outer circle in Fig. 2, and the sum S5 of intensities of spatial
frequencies 1 to 5 is a sum of intensities in the area surrounded by the inner circle in Fig.
2. In any of intensity sum calculations, the intensity of the spatial frequency 0 in the
10 center of the amplitude spectrum image is excluded. From the intensity sum S25 and
the intensity sum S5, an intensity ratio A which is a ratio (S5/S25) of the intensity sum S5
to the intensity sum S25 can be obtained.
[0048]
In the amplitude spectrum image of spatial frequencies for each gradation image
15 data item, when the intensity ratio A is relatively large, the sum S5 of intensities of
spatial frequencies 1 to 5 is large, and the amount of low spatial frequency components is
large. Accordingly, the gradation image data having a relatively large intensity ratio A
has a relatively rough satin-like appearance. On the other hand, in the spatial frequency
spectrum image for each gradation image data item, when the intensity ratio A is
20 relatively small, the sum S5 of intensities of spatial frequencies 1 to 5 is small and the
amount of low spatial frequency components is smalL Accordingly, the gradation
image data having a relatively small intensity ratio A has a relatively fine satin-like
appearance. As described above, the intensity ratio A is a parameter that can objectively
evaluate the fineness of the pattern.
25 [0049]
19
Fig. 3 shows an example of various gradation image data items and amplitude
spectrum images of spatial frequencies obtained from the gradation image data items.
In Fig. 3, the upper images are gradation image data items, the lower images are
amplitude spectrum images, and Fig. 3 shows five sets of image data items. The satin
5 pattern becomes rougher toward the right side from the left side in Fig. 3. It can be
understood that, as the satin pattern becomes rougher, the intensity at the center of the
spatial frequency spectrum image is higher, and the intensity ratio A is larger.
[0050]
In the same manner as described above, a two-dimensional discrete Fourier
10 transform is performed on gradation image data items extracted from arbitrary five areas
on the hot-dip plated layer to obtain an intensity ratio A. In addition, an average value
Aave of the obtained five intensity ratios A is obtained.
[0051]
In the hot-dip plated layer of the present embodiment, it is necessary for one or
15 more of the ratios (A/ Aave) of the intensity ratio A of each of the five measurement areas
to the average value Aave to be 0.50 to 0.65 or 1.45 to 2.00. This means that the ratio
(A/ Aave) of the intensity ratio A of each measurement area is away from the average value
Aave. Such a hot-dip plated layer has a state in which an area showing a relatively fine
satin pattern and an area showing a relatively rough satin pattern are mixed. In this
20 manner, the hot-dip plated layer in which an area showing a relatively fine satin pattern
and an area showing a rough satin pattern are mixed has a disordered appearance as a
whole, and even if physical defects such as abrasion defects and scratch marks occur on
the surface of the plating layer, the defects are less conspicuous. When the ratio
(A/ Aave) of the intensity ratio A of each of the five measurement areas to the average
25 value Aave is outside a range of 0.50 to 0.65 or 1.45 to 2.00, the appearance uniformity is
20
improved, but defects are more conspicuous. The ratio (A/Aave) may be 0.50 to 0.60 or
0.50 to 0.55. In addition, the ratio (A/Aave) may be 1.55 to 2.00 or 1.70 to 2.00. When
the ratio (A/Aave) in the five measurement areas is further from 1.00, defects are less
conspicuous.
5 [0052]
Next, a method of producing a hot-dip plated steel sheet of the present
embodiment will be described. Fig. 4 shows a hot-dip plating facility suitable for
producing the hot-dip plated steel sheet of the present embodiment. The hot-dip plating
facility shown in Fig. 4 includes a hot-dip plating bath 2, a sink roller 3 disposed in the
10 hot-dip plating bath 2, a wiping nozzle 4 disposed above the hot-dip plating bath 2, an
electromagnetic vibration device 5 disposed above the wiping nozzle 4, a cooling device
6 disposed above the electromagnetic vibration device 5, and a top roller 7 disposed
above the cooling device 6.
15
[0053]
The hot-dip plating bath 2 preferably contains Al: 2 to 22 mass%, and Mg: 0.1 to
10 mass%, with the remainder including Zn and impurities. In addition, the hot-dip
plating bath may contain Si: 0.0001 to 2 mass%. In addition, the hot-dip plating bath
may contain a total of 0.0001 to 2 mass% of one or more of the group consisting ofNi,
Ti, Zr, and Sr. In addition, the hot-dip plating bath may contain a total of 0.0001 to 2
20 mass% of one or more of the group consisting of Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc,
Y, REM, and Hf. Here, the average composition of the hot-dip plated layer of the
present embodiment is almost the same as the composition of the hot-dip plating bath 2.
The temperature of the hot-dip plating bath 2 varies depending on the composition, and is
preferably, for example, in a range of 400 to 500°C. This is because, if the temperature
25 of the hot-dip plating bath 2 is within this range, a desired hot-dip plated layer can be
21
formed.
[0054]
The electromagnetic vibration device 5 vibrates the steel sheet with a magnetic
force, and applies a magnetic force to a conveyed steel sheet 1 and vibrates the steel sheet
5 1. As the electromagnetic vibration device 5, for example, a general electromagnetic
vibration control device can be used. The electromagnetic damping device originally
prevents the steel sheet from vibrating, and includes a pair of electromagnets that are
symmetrically disposed on both sides of a traveling steel sheet 1 at a predetermined
distance (for example, 40 to 60 mm), and more desirably, two or more electromagnets on
10 one side in the plate width direction and a non-contact type steel band position detector.
The electromagnetic damping device minimizes vibration of the steel sheet by controlling
the attractive force of each electromagnet by a control device based on the detection
signal of the position detector, and can also amplify vibration by performing control
opposite to the normal control. In the present embodiment, the electromagnetic
15 damping device is used as the electromagnetic vibration device 5, and the steel sheet that
has passed through the plating bath is vibrated so that the waviness is applied in the plate
width direction. It is desirable that the electromagnetic vibration device 5 be installed
within a range of 1 m from the cooling start position (when a refrigerant is injected
toward the steel sheet, the center position at which the refrigerant is applied to the steel
20 sheet) of the cooling device 6 in the direction opposite to the traveling direction of the
steel sheet 1. That is, the device may be installed near the entrance of the cooling
device 6. It is desirable that the amplitude of the steel sheet 1 (the maximum amplitude
of the steel sheet in the cooling device) out of the electromagnetic vibration device 5 to
be 60 mm or more. In addition, it is desirable to operate the electromagnetic damping
25 device 5 in a range of 0.07 to 0.09 T.
22
[0055]
Hereinafter, a method of producing a hot-dip plated steel sheet using the
production facility in Fig. 4 will be described. First, a hot-rolled steel sheet is produced,
and as necessary, the hot-rolled sheet is annealed. After pickling, cold rolling is
5 performed to obtain a cold-rolled sheet. The cold-rolled sheet is degreased and washed,
and then annealed (the cold-rolled sheet is annealed).
[0056]
Next, as shown in Fig. 4, after the annealed steel sheet 1 is immersed in the hotdip
galvanizing bath 2, the sink roller 3 changes the traveling direction and pulls the
10 sheet up in the vertical direction. A high-pressure gas such as air and nitrogen is blown
to the surface of the pulled steel sheet 1 from the wiping nozzle 4 disposed above the hotdip
galvanizing bath 2, and thus an excessive adhesion amount of the hot-dip plating
adhered to the surface of the steel sheet 1 is removed.
15
[0057]
The adhesion amount of the hot-dip plated layer is preferably adjusted so that a
total adhesion amount on both sides of the steel sheet is in a range of 30 to 600 g/m2.
When the adhesion amount is less than 30 g/m2 , this is not preferable because corrosion
resistance of the hot-dip plated steel sheet decreases. When the adhesion amount
exceeds 600 g/m2, this is not preferable because the molten metal adhered to the steel
20 sheet drips and the surface of the hot-dip plated layer cannot be smoothed.
[0058]
Next, as shown in Fig. 4, the steel sheet 1 is vibrated by the electromagnetic
vibration device 5 and the steel sheet 1 is introduced into the cooling device 6. The
steel sheet 1 that is vibrated by the electromagnetic vibration device 5 is introduced into
25 the cooling device 6. The cooling device 6 has a built-in injection nozzle through which
23
a refrigerant is injected toward the steel sheet, and a refrigerant such as a non-oxidizing
gas or a non-oxidizing gas containing mist is injected toward the steel sheet 1 through
which the injection nozzle. Since the refrigerant is injected while the steel sheet 1 is
vibrated, the distance between the injection nozzle and the steel sheet 1 changes
5 irregularly, and thereby, the refrigerant is ununiformly applied over the entire hot-dip
plated layer, and the cooling speed on the hot-dip plated layer varies over the entire
surface of the hot-dip plated layer. Therefore, the metal structure and alloy composition
of the hot-dip plated layer after solidification vary and the appearance pattern of the hotdip
plated layer becomes disordered so that defects are less conspicuous in the
10 appearance. Here, in the conventional facility, for example, an electromagnetic
damping device may be disposed near the wiping nozzle 4 in order to minimize a
variation in the adhesion amount of the plating. In this case, since the electromagnetic
damping device is separated from the cooling device 6 and minimizes vibration of the
steel sheet 1, it is not possible to obtain an appearance in which defects are less
15 conspiCUOUS.
[0059]
According to the hot-dip plated steel sheet of the present embodiment, since at
least one ratio (A/Aave) of an intensity ratio A of each measurement area to an average
value Aave of intensity ratios A of the five measurement areas selected from the surface of
20 the hot-dip plated layer is 0.50 to 0.65 or 1.45 to 2.00, even if physical defects occur, the
defects are less conspicuous. In addition, in the hot-dip plated steel sheet of the present
embodiment, since the average composition of the hot-dip plated layer contains Al: 2 to
22 mass%, and Mg: 0.1 to 10 mass%, with the remainder including Zn and impurities, it
has excellent corrosion resistance.
25 [Example 1]
24
[0060]
Next, examples of the present invention will be described. A steel sheet after
cold rolling was degreased and washed with water. Then, the steel sheet was subjected
to cold-rolled sheet annealing. The steel sheet after annealing the cold-rolled sheet was
5 introduced into the hot-dip plating facility shown in Fig. 4, immersed in the hot-dip
plating bath and then pulled up. Then, the adhesion amount was adjusted by gas
wiping, and cooling was additionally performed. Cooling was performed by blowing a
non-oxidizing gas in the cooling device while vibrating the steel sheet with an
electromagnetic vibration device. Here, as shown in Table 1A, the electromagnetic
10 vibration device changed its position from the cooling start position (the center position
at which a non-oxidizing gas was applied to the steel sheet) in the direction opposite to
the traveling direction of the steel sheet. In Table 1A, directly below the cooling device
in the column of the vibration device position means that "vibration device is within a
range of 1 m from the cooling start position in the direction opposite to the traveling
15 direction of the steel sheet 1." In addition, Table 1A shows the maximum amplitude of
the steel sheet in the cooling device. Accordingly, Nos. 1 to 54 of hot-dip plated steel
sheets shown in Table 1A and Table 1B were produced.
[0061]
In addition, Nos. 55 and 56 of hot-dip plated steel sheets were produced in the
20 same manner as in Nos. 1 to 54 of hot-dip plated steel sheets except that the steel sheet
was not vibrated by the electromagnetic vibration device.
[0062]
For the obtained hot-dip plated steel sheet, five square areas with sides of 5 mm
on the surface of the hot-dip plated layer were selected as measurement areas, and each
25 measurement area was extracted as gradation image data with a size of 50 pixelsx50
25
pixels. The measurement area was selected as follows. At any position on the surface
of the plating layer, a square area with a side of 10 em was selected, and at a total of 5
positions, four corners of the square and the intersection of two diagonal lines of the
square, measurement square areas with sides of 5 mm were selected. The image data of
5 the measurement area was extracted by scanning the surface of the hot-dip plated layer of
the sample with a flat bed type scanner connected to a computer. The image data was
256level grayscale image data.
[0063]
Next, for each gradation image data item, a two-dimensional discrete Fourier
10 transform was performed on the gradation image data to obtain amplitude spectrum
images of spatial frequencies. In the amplitude spectrum image of spatial frequencies
for each gradation image data item, a sum S25 of intensities of spatial frequencies 1 to 25
and a sum S5 of intensities of spatial frequencies 1 to 5 were calculated, and a ratio
(S2/S25) of the intensity sum S5 to the intensity sum S25 was obtained as an intensity
15 ratio A. In addition, an average value Aave of the five obtained intensity ratios A was
obtained. Then, a ratio (A/ Aave) of the intensity ratio A of each of the five measurement
areas to the average value Aave was obtained. The results are shown in Table lB. In
Table lB, "upper left corner," "upper right corner," "lower left corner," and "lower right
corner" are a ratio (A/Aave) at the corner of a square with a side of 10 em, and "center" is
20 a ratio (A/ Aave) at the intersection of two diagonal lines of the square.
[0064]
When at least one of the ratios (A/ Aave) in the five measurement areas was 0.50
to 0.65 or 1.45 to 2.00, it was evaluated as F indicating disordered patterns and less
conspicuous defects, and when none of the ratios (A/Aave) in the five measurement areas
25 was 0.50 to 0.65 or 1.45 to 2.00, it was evaluated asP indicating uniform appearance and
26
more conspicuous defects. F was satisfactory, and P was unsatisfactory. The results
are shown in Table lB.
[0065]
In addition, the inconspicuousness of defects was visually evaluated. The
5 plating appearance was visually evaluated. After abrasion defects were made, when the
abrasion defects were not visible from 3 m ahead, it was evaluated as F indicating good
inconspicuousness of defects, and when abrasion defects were visible, it was evaluated as
P indicating insufficient inconspicuousness of defects. F was satisfactory, and P was
unsatisfactory. The results are shown in Table lB.
10 [0066]
The corrosion resistance of the hot-dip plated steel sheet was evaluated by a
corrosion weight loss after the CCT test. The plating steel sheet was cut to 150x70 mm,
and the corrosion weight loss was examined after 30 CCT cycles using CCT according to
JASO-M609. A corrosion weight loss ofless than 30 g/m2 was evaluated as F, a
15 corrosion weight loss of 30 g/m2 or more and less than 50 g/m2 was evaluated as G, a
corrosion weight loss of 50 g/m2 or more and less than 60 g/m2 was evaluated as P, a
corrosion weight loss of 60 g/m2 or more was evaluated as X. F, G and P were
satisfactory, and X was unsatisfactory. The results are shown in Table lB.
20
[0067]
As shown in Table lAand Table lB, No.1 to No. 46 of hot-dip plated steel
sheets of the examples of the present invention had less conspicuous defects and
excellent corrosion resistance.
[0068]
On the other hand, as shown in Table lA and Table lB, No. 47 to No. 56 of hot-
25 dip plated steel sheets of comparative examples had conspicuous defects or poor
27
corrosion resistance.
[0069]
In addition, regarding the evaluation of the inconspicuousness of defects, a
sufficient correlation was observed between evaluation using the ratio (A/Aave) according
5 to the present invention and visual evaluation.

[CLAIMS]
What is claimed is:
1. A hot-dip plated steel sheet comprising a steel sheet and a hot-dip plated layer
formed on a surface of the steel sheet,
wherein an average composition of the hot-dip plated layer contains Al: 2 to 22
mass%, and Mg: 0.1 to 10 mass%, with a remainder comprising Zn and impurities,
wherein, when five square measurement areas with sides of 5 mm are selected
from a surface of the hot-dip plated layer and intensity ratios A for each measurement
area are obtained by a following measurement method, at least one ratio (A/Aave) of an
10 intensity ratio A of each measurement area to an average value Aave of the intensity ratios
A of the five measurement areas is 0.50 to 0.65 or 1.45 to 2.00, and
wherein the measurement method is a method in which each measurement area
is extracted as 256 level grayscale image data with a size of 50 pixelsx50 pixels, a twodimensional
discrete Fourier transform is performed on the 256 gradation image data to
15 obtain amplitude spectrum images of spatial frequencies, and in the obtained amplitude
spectrum images, a sum S25 of intensities of spatial frequencies 1 to 25 and a sum S5 of
intensities of spatial frequencies 1 to 5 are calculated, and a ratio (S5/S25) of the
intensity sum S5 to the intensity sum S25 is defined as the intensity ratio A.
20 2. The hot-dip plated steel sheet according to claim 1,
wherein the average composition of the hot-dip plated layer contains Al: 4 to 22
mass%, and Mg: 1.0 to 10 mass%.
3. The hot-dip plated steel sheet according to claim 1 or 2,
25 wherein the average composition of the hot-dip plated layer further contains Si:
35
0.0001 to 2 mass%.
4. The hot-dip plated steel sheet according to any one of claims 1 to 3,
wherein the average composition of the hot-dip plated layer further contains a
5 total of 0.0001 to 2 mass% of one or more of a group consisting ofNi, Ti, Zr, and Sr.
5. The hot-dip plated steel sheet according to any one of claims 1 to 4,
wherein the average composition of the hot-dip plated layer further contains a
total of 0.0001 to 2 mass% of one or more of a group consisting of Fe, Sb, Pb, Sn, Ca,
10 Co, Mn, P, B, Bi, Cr, Sc, Y, REM, and Hf.
6. The hot-dip plated steel sheet according to any one of claims 1 to 5,
wherein an adhesion amount of the hot-dip plated layer i s a total of 30 to 600 g/m2
on both sides of the steel sheet.